Area Forecast Discussion
Issued by NWS Sterling, VA

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910 FXUS61 KLWX 230117 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 917 PM EDT Wed Jun 22 2016 .SYNOPSIS... Low pressure will approach from the Ohio Valley tonight and then move across the region Thursday. High pressure will build into the area Thursday night and remain through the weekend. A cold front will approach the mid-Atlantic early next week. && .NEAR TERM /THROUGH THURSDAY/... At present...a frontal boundary remains draped across central Virginia. Much drier and more stable air mass in place across most of the region at present. (The 00z LWX RAOB has 0 CAPE, while the sounding from RNK has SB/MU CAPE almost 1750 j/kg.) It still looks like the front will try to move back northward in advance of low pressure moving eastward from the midwest. This low will pass near the area during the day Thursday. There is still more than the desired uncertainty regarding severe weather and flooding, mainly due to the presence of the aforementioned front. Some guidance (12Z GFS in particular) shoves the front back north late tonight and early Thursday, resulting in an extremely unstable atmosphere on Thursday afternoon (4000+ j/kg cape) from which strong to severe storms would be likely. However, this would not seem particularly likely if the idea of an MCS moving across the region is correct...in which case the rain cooled air would likely be stuck in a wedge east of the mountains and the front would be unable to push back north. The other question revolves around said MCS, which may or may not bring a round of severe weather later tonight. With the front sagging southward still and a stable air mass in place, any MCS moving in from the west could just ride right over the stable air mass and start weakening, which would reduce the severe threat. However, should the front manage to slide back northward late tonight ahead of the MCS, it could tap the more unstable air to the south directly and bring another round of severe thunderstorms and heavy rain. Right now (01Z), it appears as though the MCS is organizing in northern Illinois. It has a long way to travel (nearly 600 mi) before it affects the east coast. Most of the recent HRRR/RAP/LWX WRF-ARW4 runs as well as the 18z GFS/NAM all suggest precipitation arriving in the Applachians somewhere around 09z, and to I-95 near 12Z (give-or-take). That is a logical progression based on radar trends. Have therefore pulled all mention of rain from the forecast for the evening, and reintroduced it toward morning. It remains to be seen how much instability will be had at that hour, or whether the in-storm environment will render that a moot point. Can`t rule out heavy rain or gusty winds...but am delaying them a bit. The Flash Flood Watch remains in place for the Potomac Highlands. These areas have low flood guidance values and have the highest certainty of impact prior to potential weakening. Beyond that, it`s not completely clear whether the complex will track east or dive south. Therefore, am not making any changes at this time. Jury is still out with respect to Thursday; the early morning activity will play a role. However, it seems as though the focus /may/ be on our Virginia counties as opposed to Maryland. But, that depends on what happens to the front behind the likely- decaying MCS as well as how much recovery transpires...so there is plenty of room for that projection to change. Made no changes to the temperature forecast at this time. Lows tonight will be mostly in the 60s. Highs on Thursday are uncertain but are forecast mainly in the upper 70s and 80s. Depending on where the front ends up...this forecast could be off by some degree. && .SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... Front drops south of the area Thursday night and remains south through Saturday. High to the north will provide northeast to east flow across the region in its wake. Upslope flow combined with trough aloft may allow isolated to scattered showers and perhaps even a stray thunderstorm mainly near the mountains...and especially further south. Otherwise it will be rather pleasant. The threat of showers will diminish Saturday. && .LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/... 500 MB height rises through Sunday night before zonal flow sets up through the Monday night. H5 trough and associated surface cold front approaches late Monday with frontal passage in the Tuesday/Tuesday night time frame. Best chances of shower/thunderstorms will be from Monday afternoon through Tuesday evening. Maximum/minimum temperatures and POPs are close to normals for early summer. No outstanding heat or humidity issues in the forecast at this moment. && .AVIATION /01Z THURSDAY THROUGH MONDAY/... Still big uncertainty early Thursday regarding severe weather threat...but at least through midnight we should generally stay VFR at all terminals. The only substantive change was to delay the primary threat window closer to 12z. Early morning thunderstorms can bring reductions in vis and cigs to IFR...along with damaging winds. There may or may not be a second round Thursday afternoon. It will depend on the track/timing of the morning convective complex, the positioning of the stalled boundary behind the complex, and how much recovery we can experience during the middle of the day. Improving conditions generally Thursday night and VFR Friday - Sunday. MVFR conditions possible early next week for all terminals with cold frontal passage. && .MARINE... Winds overnight will be light. Storm complex moving in from the west will bring rain, thunderstorms and another round of SCA winds on Thursday. Conditions improve Friday-Saturday. In the extended time frame, best chances of small craft conditions will be on Monday pre-frontal, and then again on Tuesday post- frontal. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...Flash Flood Watch from 2 AM EDT Thursday through Thursday afternoon for MDZ003-501-502. VA...Flash Flood Watch from 2 AM EDT Thursday through Thursday afternoon for VAZ027-028-030-031-503-504. WV...Flash Flood Watch from 2 AM EDT Thursday through Thursday afternoon for WVZ050>053-055-501>506. MARINE...Small Craft Advisory from 6 AM to 6 PM EDT Thursday for ANZ530>543. && $$ SYNOPSIS...Lee NEAR TERM...RCM/HTS SHORT TERM...RCM LONG TERM...Lee AVIATION...HTS/RCM/Lee MARINE...HTS/RCM/Lee

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